Crude oil heat exchange

ABSTRACT

In the method of retarding corrosion of a heat exchanger being used to cool hot hydrocarbon vapors from a crude column by adding water to said vapors, the improvement to reduce loss of heat available in said exchanger by adding said water only for a time of from about 0.5 to about 2 hours within a 24 hour period.

[ Nov. 20, 1973 CRUDE OIL HEAT EXCHANGE [75] Inventor: Russell F.Stedman, Des Plaines, 111.

[73] Assignee: Universal Oil Products Company, Des Plaines, Ill.

[22] Filed: Jan. 24, 1972 [21] Appl. No.: 220,455

[52] US. Cl 208/47, 165/1, 165/134,

203/7, 208/48 AA, 208/348 [51] Int. Cl Cl0g 9/12, C10g 9/16 [58] Fieldof Search 208/47; 203/7; l65/95,1, 134

[56] References Cited UNITED STATES PATENTS 2,938,851 5/1960 Stcdman eta1. 208/47 2,499,435 3/1950 Whitacre 208/47 3,024,171 3/1962 Bone 208/47X 2,414,761 1/1947 Nutting et al 203/7 12/1950 Friedman 203/7 X 8/1960Phillips 203/7 UX OTHER PUBLICATIONS Parker, Ivy M., National PetroleumNews, Refinery Corrosion Problems Reviewed, Sec. 2, June 7, 1944, p.R-376.

Primary Examiner--Albert W. Davis, Jr. Assistant Examiner-S. J. RichterAttorney.lames R. Hoatson, Jr. et a1.

[57] ABSTRACT In the method of retarding corrosion of a heat exchangerbeing used to cool hot hydrocarbon vapors from a crude column by addingwater to said vapors, the improvement to reduce loss of heat availablein said exchanger by adding said water only for a time of from about 0.5to about 2 hours within a 24 hour period.

6 Claims, No Drawings BACKGROUND OF THE INVENTION In the processing ofcrude oil, general practice is to first subject the crude oil to adesalting treatment. The desalted oil then is subjected to steamstripping in a crude column, with the vapors being taken overhead andthe heavier products being withdrawn as side streams and bottomsproduct. The overhead vapors from the crude column are cooled andcondensed by first being passed into indirect heat exchange with thecrude oil charge and then by further cooling and condensing.

While much of the corrosive components of the crude oil are removed inthe desalting operation, the crude oil still contains some metalchlorides (particularly calcium chloride and magnesium chloride) whichdecompose to release hydrogen chloride. In order to neutralize theacidic corrodents, it is general practice to introduce ammonia,morpholine or other basic reagent into'the crude column overhead vaporline. Preferably a basic corrosion inhibitor, particularly of the film.forming type, also is introduced into the vapor line. To still furtherreduce corrosion, it is general practice to continuously introduce waterinto the vapor line ahead of the heat exchanger, thereby to flush downammonium chloride which is present in the overhead vapor line. Oneimprovement in this method of (meta tion is the reuse of the watercollected in the receiver into which the condensed vapors pass.

DESCRIPTION OF THE INVENTION While the continuous introduction of waterinto the overhead vapor line serves to reduce corrosion in the heatexchanger and communicating equipment, this procedure has thedisadvantage of reducing the'amount of heat available in the exchanger.As hereinbefore set forth, the incoming crude oil charge is passed intoindirect heat exchange with the hot vapors from the crude column. Theheated crude oil charge then is supplied to conventional desaltingoperation. It is important that the temperature of the desalting besufficiently high to accomplish efficient desalting of the crude oil.Accordingly, for economical purposes, it is desirable that the heatavailable in the heat exchanger be kept at a maximum so that such heatmay be used to heat the incoming crude oil to a sufficiently hightemperature for effective desalting treatment. Otherwise it may benecessary to utilize other means to obtain such heating and r such othermeans entail additional equipment and cost.

The present invention is based on the principle that sufficient removalof acidic corrodents may be effected by only a part-time flushing withwater. It is believed that corrosion in the heat exchanger and followingequipment is caused by the deposition of solid ammonium chloride, whichoccurs upon cooling of the vaporous acidic corrodents. One theory isthat ammonium chloride catalyzes sulfide corrosion, apparently throughthe reaction of the ammonium chloride with the sulfur containingcompounds to form some ammonium sulfide compound or complex which; inturn, forms a first stage iron sulfide type corrosion and, if notremoved, continues to corrode the metal. It is understood that thepresent applicant does not wish to be limited to this explanation but itdoes appear to be a reasonable explanation of the mechanism.

As hereinbefore set forth, upon cooling, ammonium chloride solidifiesand, if allowed to remain, causes corrosion of the heat exchanger tubes.Generally, the overhead vapor fraction is withdrawn at a temperature offrom about 250 to about 350 F. and is progressively cooled in the firstexchanger to from about 200 to about 300 F. and in the second heatexchanger to from about to 250 F. While additional heat exchangers maybe used, in most cases one or more water cooled condensers are utilizedto reduce the temperatureof the vapors down to about ambienttemperature. It is understood that these temperature ranges are typicalonly, and will vary with the specific procedure employed at a particularrefinery.

As hereinbefore set forth, it is general practice to continuously flushthe ammonium chloride down through the heat exchangers and condensers byintroducing water at the inlet of the first heat exchanger. Accordingly,the water contacts the ammonium chloride before it is allowed tosolidify. However, in accordance with the present invention it isbelieved that substantially the same objective is accomplished byintroducing the water for only a comparatively short time instead ofcontinuously as heretofore believed necessary. This is based upon therecognition that ammonium chloride undergoes very slow deposition and,by the periodical flushing herein proposed, will be washed out of theheat exchangers and condensers sufficiently to avoid corrosion of thesteel equipment. I

From the above discussion, it will be seen that the advantages of thewater flushing of the acidic corrodents is retained without thedisadvantages of reducing the heat available for use to heat theincoming crude oil charge. In other words, instead of the continuousloss in available heat, such loss is encountered for only a short periodof time. This novel method offers an important economical advantage inthe operation of the process.

The water to be introduced into the overhead vapor line may come from anoutside source or conveniently is water collected 'in the receiver andreused for this purpose. As hereinbefore set forth, conventionalpractice is to pass the hot hydrocarbon vapors down through one or aplurality of heat exchangers, then through one or a plurality of watercondensers and fi-.

nally collected in a receiver. Because the crude column normally issubjected to water stripping, the overhead vapors contain the waterwhich ultimately is collected in the receiver. Because this-water doesnot contain undesirable impurities, as for example oxygen, reuse of thiswater for this purpose is particularly desirable. Accordingly, there isa large volume of such water available for such reuse.

The amount of water to be commingled with the crude columnoverheadvapors will be determined by the amount necessary to accomplishsatisfactory removal of the acidic'corrodents. As a practical matter,the amount of this water will be determined by the pumping and pipingcapacity available at the particular refinery. In any event, theintroduction of the water will be limited for a timeiof from about 0.5to about 2 hours and still more particularly from about 0.75 to about1.5 hours within a 24 hour periodJExcess water from the receiver iswithdrawn from the system and, as another advantage to the process ofthe present invention, the withdrawal of the excess water also serves toremove from the system the dissolved ammonium chloride and other acidiccorrodents.

In view of the fact that the desalting and crude column operations arewell known in the industry, there appears no need to describe suchwell-known procedures in detail in the present application. For example,R. H. Carlton US. Pat. No. 3,189,537 describes the continuous reuse ofwater collected in the receiver and the drawing illustrates a typicalsystem. The drawing and description thereof in this patent, as well asthe discussion of suitable film forming inhibitors and other details ofoperation, may be considered as the prior art and embodied in thepresent application as to details not repeated herein in the interest ofbrevity.

The following examples are introduced to illustrate further the noveltyand utility of the present invention but not with the intention ofunduly limiting the same.

EXAMPLE I In this example, the crude column is connected to two heatexchangers and two condensers. Steam is introduced into the lowerportion of the crude column at a rate of 2250 pounds per hour. Theoverhead vaporous fraction is withdrawn at a rate of 7500 barrels perday and at a temperature of 275 F. and a pressure of 9 p.s.i.g. Ammoniais introduced into the vapor line at a rate of 50 pounds per day and acorrosion inhibitor is introduced at a concentration of 20 parts permillion based on the overhead vapor. The inhibitor is introduceddownstream from the ammonia injection, but before the mixture is passedinto the first heat exchanger. The inhibitor is a dibasic acid ofa highmolecular weight alkyl amine. In the first heat exchanger, the vaporizedoverhead is cooled to about 240 F. and further cooled in the second heatexchanger to a temperature of about 200 F. Crude oil being charged tothe process is passed successively through the second heat v exchangerand the first heat exchanger, after which it is further heated and thensent to the desalter. The overhead effluent, after passing through thesecond heat exchanger, is finally cooled in the condensers to atemperature of 90 F. and is collected at this temperature and a pressureof 5 p.s.i.g. in the receiver.

In accordance with the present invention, water separated in thereceiver is commingled with the overhead vapors prior to solidificationof ammonium chloride. The water is introduced into the vapor line aheadof the first exchanger. The reuse water is supplied at a rate of 30gallons per minute for a period of 45 minutes during each 24 hours.Excess water containing dissolved ammonium chloride and other acidiccorrodents is withdrawn from the receiver and utilized for any desiredpurpose or disposed'of in any suitable manner.

EXAMPLE II In this example the crude column is preceded 'by apreflashing system. The preflash crude is supplied to the crude columnwhich is maintained at a pressure of 14 p.s.i.g. Steam is introduced tothe crude column at a rate of 1000 pounds per hour. A naphtha overheadis removed at a rate of 9400 barrels per day and a temperature of 310 F.Ammonia is introduced into the upper portion of the crude column at arate of pounds per day and a corrosion inhibitor is introduced at aconcentration-of 30 parts per million'based upon the overhead vapor. Theoverhead vapor is withdrawn at a temperature of 310 F. and then iscooled in a heat exchanger in which the overhead vapor is passed inindirect heat exchange with the crude oil. In the heat exchanger, theoverhead vapor is cooled to a temperature of 250 F. and then is furtherpassed through two condensers and finally collected in the receiver at atemperature of F. Prior to the use of the reuse water, corrosion wasobserved in the heat exchanger.

Water from the receiver is injected ahead of the heat exchanger at arate of 40 gallons per minute for a time of 60 minutes in each 24 hourperiod. Excess water is removed from the receiver and utilized asdesired or disposed of in any suitable manner. l

The reuse water supplied in the manner described above serves to retardcorrosion of the carbon steel heat exchanger tubes.

I claim as my invention:

1. In the method of retarding corrosion of a heat exchanger being usedto cool hot ammonium chloridecontaining hydrocarbon vapors from a crudecolumn by adding water to said vapors, the improvement to reduce loss ofheat available in said exchanger, which comprises adding said waterprior to solidification of the ammonium chloride and only for a time offrom about 0.5 to about 2 hours within a 24 hour period.

2. The method of claim 1 in which incoming crude oil is passed intoindirect heat exchange with said hydrocarbon vapors.

3. The method of claim 1 in which said water is added only for a time offrom about 0.75 to about 1.5 hours within a 24 hour period.

4. The methbd of claim 1 in which said water is at least a part of thewater collected in the receiver into which the cooled hydrocarbon vaporspass.

5. The method of claim 1 in which said water is added to the hydrocarbonvapors prior to entering theheat exchanger.

6. The method of claim 5 wherein a plurality of heat exchangers andcondensers are used in the cooling and condensing of said hothydrocarbon vapors, and the water is injected ahead of the first heatexchanger.

ma s-us

2. The method of claim 1 in which incoming crude oil is passed intoindirect heat exchange with said hydrocarbon vapors.
 3. The method ofclaim 1 in which said water is added only for a time of from about 0.75to about 1.5 hours within a 24 hour period.
 4. The method of claim 1 inwhich said water is at least a part of the water collected in thereceiver into which the cooled hydrocarbon vapors pass.
 5. The method ofclaim 1 in which said water is added to the hydrocarbon vapors prior toentering the heat exchanger.
 6. The method of claim 5 wherein aplurality of heat exchangers and condensers are used in the cooling andcondensing of said hot hydrocarbon vapors, and the water is injectedahead of the first heat exchanger.